This invention relates generally to medical diagnostic ultrasound systems and more particularly to high-PRF pulsed-doppler ultrasound methods and apparatus.
Sound waves having a frequency greater than approximately 20 kHz are referred to as ultrasound. In medical diagnostic applications, ultrasound signals are transmitted into a patient's body where they are in-part absorbed, dispersed, refracted and reflected. Reflected ultrasound signals are received at transducer elements which convert the reflected ultrasound signals back into electronic signals.
A final ultrasound beam-pattern, to the first order, is a product of a transmit beam-pattern and a receive beam-pattern. The final beam-pattern typically is processed to analyze echo, doppler and flow information and obtain an image of the patient's encountered anatomy (e.g., tissue, flow, doppler). Diagnostic sonography refers to the medical cross-sectional anatomic and flow imaging derived from pulse-echo ultrasound signals. Doppler ultrasound is the field of detection, quantization, and medical evaluation of tissue motion and blood flow. Continuous wave doppler ultrasound (i.e., cw-doppler) uses continuous-wave ultrasound signals. Pulsed doppler ultrasound uses pulsed-wave ultrasound signals.
This invention relates to pulsed-wave doppler ultrasound. Applications of doppler ultrasound are found in virtually all medical specialties, including cardiology, neurology, radiology, obstetrics, pediatrics and surgery. Flow can be detected even in vessels that are too small for sonographic imaging. Doppler ultrasound is used to determine the presence or absence of flow, the direction and speed of flow, and the character of flow. Doppler instruments typically provide both audible and visual outputs indicative of blood flow information. CW-doppler instruments are used to monitor a large sample volume, but can give complicated and confusing presentations if reflectors or scatterers with different motions or flows are included in the volume. Pulsed doppler systems address this difficulty by detecting motion or flow at selected depths within a relatively small volume (i.e., a range gate).
Doppler ultrasound is based upon the doppler effect, which is a change in frequency caused by the relative motion among a wave source, receiver and reflector. As applied to medical applications, an ultrasound transducer embodying the source and receiver is stationary, while blood or tissue fluid is the moving reflector. The change in frequency detected is the difference between the transmitted ultrasound signal frequency and the reflected ultrasound signal frequency. Such change is a function of the transmitted signal frequency, the propagation speed of the transmitted signal through the patient's anatomy, the speed of flow in the range gate and the angle of incidence between the ultrasound signal and the direction of blood flow.
Pulsed doppler ultrasound instruments emit ultrasound pulses and receives echoes using typically a single element or array transducer. Using range gating, pulsed doppler provides the ability to select information from a particular location (e.g., depth) within the anatomy along the beam. Typically pulsed doppler is combined with real-time sonography imaging for medical diagnostic applications.
A pulsed doppler instrument includes a voltage generator (e.g., oscillator) with an oscillator gate that generates electrical signal inputs to a transducer. The oscillator gate allows respective pulses of several voltage cycles to pass to the transducer for conversion into respective ultrasound pulses. Ultrasound pulses used for doppler have minimum pulse lengths of approximately five cycles and typical pulse lengths of 25-30 cycles. The multiple cycles within a pulse are used to determine the doppler shift of returning echoes. Voltage pulses resulting from received echoes are processed in a receiver, where they are amplified and compared in frequency with the transmitted signal. Also, a doppler shift is derived and sent to loudspeakers and a visual display.
A range gate is defined for selecting the echoes corresponding to a given depth. Specifically, a sample volume is defined as the range gate. Echoes arrive from the sample volume at a rate determined by a pulse repetition frequency ("PRF"), (i.e., number of pulses transmitted per second). Each of these returning echoes yields a sample of the Doppler shift. The samples are processed, connected and filtered to derive a sample waveform. The range gate selects a listening region from which the returning echoes are accepted. The width and height of the range gate is determined by the width and height of the beam. The length of the range gate is determined by the pulse length, (i.e., one half the pulse length is added to the gate length to yield a sample volume length). Larger range gate lengths (e.g., 10 mm) are used when searching for a desired vessel or flow location. Shorter lengths (e.g., approximately 2 mm) are used for spectral analysis and evaluation. The shorter range gate length improves signal-to-noise ratio and the quality of a spectral trace. A single range gate permits only one depth and length selection at a given time.
The echoes sensed at a range gate undergo spectral analysis of frequency components. Typically, several frequency components are present. If all flow within the range gate were of a uniform speed and direction, then there would only be one frequency component. The character of flow in vessels, however, is determined by the vessel size and the uniformity of its walls. Changes in size, turns and abnormalities, such as the presence of plaques and stenoses, alter the character of the flow. Conventionally, flow is characterized as plug, laminar, parabolic, disturbed, and turbulent. Accordingly, portions of flow often are moving at different speeds and, sometimes, in different directions. Thus, many different doppler shifts, and thus frequency components, occur. Typically, the doppler response undergoes fast Fourier transform analysis to derive the component frequencies.
In certain instances, artifacts occur in doppler ultrasound. Artifacts as used herein are anything that is not properly indicative of the structures of the flows imaged or sampled. More specifically, artifacts are incorrect presentations of flow or image information. Artifacts are caused by some characteristic of the sampling or imaging technique. Although other imaging and doppler artifacts occur, addressed here are two common doppler ultrasound artifacts--aliasing and range ambiguity.
Aliasing is the improper representation of information that has been insufficiently sampled. The sampling can be of a spatial or temporal nature. As the sampling rate is reduced, for example, the ability to resolve the details of an object, then the general character of the object, is lost. In cases this results in the object being mis-characterized (e.g., having a false appearance or assumed identity--an alias). An example of temporal aliasing occurs in, for example, a rotating object such as a fan. The blades of the fan are observed to rotate at various speeds and in reverse directions when viewed with a strobe light flashing at various rates.
The Nyquist limit or Nyquist frequency describes the minimum sampling rate required to avoid aliasing. Specifically, there must be at least two samples per period of the wave being observed. For a complicated signal, such as a doppler echo signal containing many frequencies, it is preferable that the sampling rate be sufficient to include at least two samples for each period of the highest doppler-shift frequency present. Stated differently, if the highest doppler-shift frequency present in a signal exceeds one half the pulse repetition frequency, then aliasing occurs. On a doppler spectral display, frequency aliasing is manifested as a "wrapping around" of the spectrum so that blood of high velocity in one direction instead appears to be going in the opposite direction. To reduce aliasing, the pulse repetition frequency (PRF) often is increased.
The range ambiguity artifact often is encountered in attempting to solve the aliasing problem, (e.g., when increasing the PRF). Range ambiguity occurs when a pulse is emitted before all the echoes from the preceding pulse have returned. In such cases, the late echoes from the prior pulse are received contemporaneously with early echoes from a current pulse. As a result, the doppler instrument is unable to determine whether the echo is an early echo from the current pulse or a late echo from the previous pulse. Typically, the instrument assumes that all echoes are derived from the current pulse--and attributes a corresponding depth to each respective echo. For high pulse repetition frequencies, such assumption is invalid. In such instances Doppler information is coming from locations other than the sample volume (i.e., the gate location). In effect, multiple gates are occurring at different depths. The multiple range gates are referred to as a real or primary range gate and one or more phantom or secondary range gates. The primary range gate is the sample volume of interest. The phantom range gates create range ambiguity.
In countering the aliasing and range ambiguity artifacts, one desires to increase the pulse repetition frequency to avoid aliasing, but not increase it so much that range ambiguity occurs. In practice, frequency aliasing sets a lower bound for the pulse repetition frequency, while range ambiguity sets an upper bound. Equation I defines the aliasing bound. Equation II defines the range ambiguity bound. EQU f.sub.R &gt;2 f.sub.Dmax (I) EQU f.sub.R &lt;c/2d.sub.max (II)
Where
f.sub.R =pulse repetition frequency; PA1 f.sub.Dmax =highest doppler shift that might be encountered; PA1 c=propagation speed of ultrasound in tissue; and PA1 d.sub.max =maximum depth for receiving echoes.
Resolving equations I and II gives: EQU f.sub.Dmax *d.sub.max &lt;c/4 (III)
When equation III is true, pulsed doppler is effective to avoid aliasing and range ambiguity. Unfortunately, neither f.sub.Dmax nor d.sub.max are known in advance. When nature is not cooperative, a compromise solution typically is adopted. In practice, one either (a) uses a high pulsed repetition mode in which multiple gates are observed, or (b) uses cw-doppler where range discrimination is completely sacrificed in the interest of avoiding frequency aliasing. This invention is directed toward the first alternative--use of a high PRF. The term high-PRF pulsed-doppler as used in this application refers to doppler ultrasound signals which do not conform with equation II, (i.e., signals having a pulse repetition frequency, f.sub.R, greater than or equal to the range ambiguity bound, c/2d.sub.max). More specifically, this invention is directed toward avoiding, or reducing the impact of range ambiguity in the presence of phantom gates.